The invention relates to a forward error correction (FEC) arrangement for multipoint to single point communication systems such as discrete wavelet multitone (DMT(DWMT)) systems and time division multiplexed (TDM) systems.
Conventional FEC methods use a combination of data interleaving and coding (Reed-Solomon codes are the most frequently used) to achieve protection against burst error events, e.g. impulse noise. At the transmit side of a communication system, data is scrambled into an interleaving RAM and coded. At the receiver side, the data is descrambled after error correction. The scrambling of the data has the effect of spreading the errors due to burst error events along many code words so that they can be corrected by using codes with a reduced error correction capability.
Many interleaving methods have been developed over the years. The oldest are the block interleaving, systematic (code) interleaving and convolutional (Ramsey type) interleaving. Recently, new methods have been proposed in conjunction with ADSL/VDSL applications.
Finding an efficient solution for FEC with interleaving in the case of multipoint to single point transmission systems is particularly difficult. One evident solution is the use of a FEC interleaving scheme for each multiuser as shown in FIG. 1. A 3-to-1 communication system 100 includes data input lines 102.sub.1 -102.sub.3, interleavers 104.sub.1 -104.sub.3, RS coders 106.sub.1 -106.sub.3, and transmitters 108.sub.1 -108.sub.3 on the transmit side. The data is transmitted over a data channel 110, e.g., twisted pair, to a receiver 112. The receive side of system 100 includes RS decoders 114.sub.1 -114.sub.3, deinterleavers 116.sub.1 -116.sub.3, and output data lines 118.sub.1 -118.sub.3.
There are three components which are expensive to implement in every FEC/interleaver system. The components are the interleaver RAM at the transmit side, and the deinterleaver RAM and the Reed-Solomon decoder at the receive side. One additional problem arises when the data rate from each multiuser is adjusted according to the multiuser individual needs. In these cases, the size of the interleaver/deinterleaver RAM for each multiuser should accommodate the maximum data rate. This results in large interleaver and deinterleaver RAMs for each user, which are seldom used at full capacity.